CM41xx: fix signedness of env bootargs string pointer

The pointer to the flash based bootargs should be a "char *", not unsigned.
Fixes:

cm41xx.c: In function ‘env_flash_cmdline’:
cm41xx.c:67: warning: pointer targets in passing argument 2 of ‘setenv’ differ in signedness

Signed-off-by: Greg Ungerer <greg.ungerer@opengear.com>